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Data are presented on the results of geobotanical studies along a 27.7-km permanent transect across the northern Volga–Akhtuba floodplain on which 88 geobotanical sampling plots are located. The transect was established in 1955, before the largest hydroelectric complexes were built on the Volga. As a result of their operation, the annual average floodwater discharge to the Volga–Akhtuba floodplain has decreased by 10–15% since 1959, amounting to 40–45% of the total annual water discharge. The water released from the Volgograd Reservoir falls from high height, has much kinetic energy, and its impact leads to deepening of the Volga riverbed in the northern part of the floodplain. Therefore, the same volumes of releases from the reservoir during floods result in increasingly lower rise in water level and, as a consequence, reduction in the inundated floodplain area. The results of surveys in 1982, 2008, and 2019 showed that xerophytization of vegetation in the floodplain region crossed by the transect has progressed during the 64-year period. Changes in the vegetation caused by the impairment of moisture conditions in the floodplain are accompanied by successions of phytocenoses resulting from invasion by new species. Among them, the North American species Fraxinus pennsylvanica has the most significant effect on the natural vegetation. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Tritium and carbon-14 used to assess pattern of Murray River floodplain recharge. • Pike floodplain recharged via inundation through surficial clay unit. • Katarapko floodplain recharged via riverbank recharge. • Results critical to maintaining health of floodplain ecosystems. Abstract Floodplains may alternate between discharge zones for regional groundwater and areas of recharge from river water during high-flow events. Understanding the mechanisms and timescales of recharge on floodplains is important for their management and for the protection of fragile ecosystems. The floodplains of the River Murray host important ecosystems, particularly remnant eucalypt forests that are vulnerable to changes in inundation, rising regional water tables, and salt accumulation. This study addresses floodplain recharge and groundwater mixing on the Pike and Katarapko floodplains of South Australia. At Pike, 3H activities of groundwater in the low hydraulic conductivity Coonambidgal Formation that crops out on the floodplain are ∼0.25 TU. 3H activities of groundwater in the underlying higher hydraulic conductivity Monoman Formation decrease from ∼1 TU near the contact with the Coonambidgal Formation to <0.02 TU at >15 m depth. Groundwater 14C activities are between 40 and 95 pMC but are less well correlated with depth. The 3H or 14C activities do not vary systematically with distance from the surface water channels on the floodplains. These observations imply that groundwater recharge at Pike is dominantly through the floodplain rather than through the channel banks. In contrast to the regional groundwater where total dissolved solids (TDS) concentrations are commonly >35,000 mg/L, the TDS of groundwater on the floodplain is locally <500 mg/L. A correlation between 3H activities and TDS and the presence of groundwater with relatively low 14C activities but above detection 3H activities implies that recently recharged waters have mixed with regional groundwater in the floodplain sediments. The 3H activities in these mixed waters implies that mixing occurs over a few years. By contrast, at Katarapko, the highest 3H activities in the Monoman Formation groundwater (up to 2.35 TU) are closer to the Murray River, implying that recharge through the bank may occur. These contrasting patterns of recharge probably reflect local topographic controls. Understanding the recharge-discharge relationships are vital for managing proposed floodplain inundation programs aimed at improving ecosystem health. [ABSTRACT FROM AUTHOR]

Floodplains are ecotones that form a transition between aquatic and terrestrial environments. These important ecosystems can be described as dynamic spatial mosaics in which water plays an important role in connecting various patches on the floodplain surface. Hydrological connections facilitate the exchange of carbon and nutrients between the river channel and the floodplain and therefore influence the productivity of the entire river system. This paper examines the influence of hydrological connections on the potential exchange of dissolved organic carbon between a large Australian floodplain to a river channel, and the effects of land and water developments on these exchanges. The paper proposes that an understanding of floodplain ecosystems requires an interdisciplinary approach—a recognition of the importance of the three disciplines hydrology, geomorphology and ecology. Large-scale water-resources and floodplain development has significantly altered the spatial and temporal patterns of hydrological characteristics in the Lower Balonne floodplain, Australia. The magnitude, frequency and duration of flooding events have all been reduced. The construction of levees and water storages has also reduced the reactive floodplain surface area. The presented data show the impacts of these changes on the potential supply of dissolved organic carbon from the floodplain surface during periods of inundation. Annual reductions of up to 1293 tonnes of dissolved organic carbon supply were noted and reductions were especially significant for floods with an average recurrence interval of 2 years or less. Some small flood events no longer facilitate the potential supply of dissolved organic carbon from the floodplain to the river channel because of water-resources and floodplain developments. [Copyright &y& Elsevier]